1. Field of the Invention
This invention relates to suppression of seismic multiple reflection signals, and more particularly to a method and system for modeling land-based seismic wave-field data.
2. Description of Related Art
Exploration for subterranean natural resources, such as oil and gas, commonly employs the generation of controlled seismic signals, gathering reflected signals, and processing the reflected signals and to yield information about the subterranean formations. The reflected signals, known as “seismic reflections,” or simply “reflections,” are detected by sensors or receivers typically located at or near the earth's surface, in an overlaying body of water, or at known depths in boreholes. The information from reflections is derived from what are known as “primary” reflections, which are acoustic reflections caused by subsurface anomalies, e.g., variations in elastic properties of rock formations or other underground features. The seismic data is recorded and processed to output mappings of an area of interest, including three-dimensional representations of the geological formations.
Collection of seismic data by receivers occurs over a period of about 1 to about 12 seconds immediately following generation of a down-going seismic energy pulse, such as from one or more detonated explosions or seismic vibration sources. The data is collected over intervals of one to about four milliseconds. Therefore each receiver can collect thousands of returning seismic recordings related to a down-going seismic energy pulse. The data collected is commonly known as “seismic traces.” In a typical seismic survey, which can involve thousands of individual receivers at known locations, millions of seismic traces are received, recorded and processed over a period of about six seconds.
Each seismic trace not only includes the desired primary reflections, but also includes random noise and undesirable “multiple reflection signals,” also know as “multiples.” Multiple reflections occur when a seismic wave reflects up at the location of a subsurface anomaly, reflects back down, and back up again. Further, multiples often reflect more than three times. Random noise and multiples obscure the primary reflections, thus making the seismic data very difficult to interpret, even by experienced geophysicists. Therefore, numerous techniques have been developed to minimize the impact of the unwanted data.
Conventional methodologies for providing usable data from reflection data are to input the collective data, including random noise, primary reflections and multiple reflections into a full data transform and convert it to another domain. However, in a full data transform, if the noise energy is excessive, it is difficult to mute only multiples in a computer-generated model. Since every sample in model space is a combination of noise, primaries and multiples, when a portion of a sample in model space is muted, all noise, primaries and multiples for all traces will be impacted accordingly.
The most utilized methods for multiples suppression are Radon filter-based methodologies. These are described, for example, by Moore, et al. in U.S. Pat. No. 6,636,810; Beylkin in U.S. Pat. No. 4,760,563; Gasparotto, et al. in U.S. Pat. No. 6,094,620; Sacchi in “Fast high resolution parabolic Radon transform,” SEG Expanded Abstracts (1999); Cary in “The simplest discrete Radon transform,” SEG Expanded Abstracts (1998); Schonewille et al. in “High-resolution transform and amplitude preservation,” EAGE 64th Conference & Exhibition, Florence, Italy (2002); and Cao et al. in “A Semblance Weighted Radon Transform on Multiple Attenuation,” CSEG National Convention (2005). All of these references describe modeling of multiples in full data multi-channel transforms.
The Radon-based methodologies do not account for special characteristics associated with land data. Land data, unlike marine data, is characterized by multiple arrivals that are neither continuous nor predictable due to near-surface and sub-surface anomalies. Radon transforms, along with Fourier and several other transforms, are typical tools available to geophysicists for modeling multiples. Land data is characterized with low signal-to-noise ratio. Unlike the marine environment, land has variable surface conditions which lead to the generation of various multiple events. Further, land data has a wide variety of noise, and land multiple are neither continuous nor predictable. Therefore, while multi-channel multiple suppression methods may be suitable for modeling marine-based seismic data, these methodologies tend to cause data smearing, which in turns lead to changes in the original texture of the seismic data. This is due to the fact that traditional methods use multi-channel transforms and model multiples directly from the low signal-to-noise ratio input data.
Therefore, a need exists for a method and system for modeling multiple reflections that is particularly suitable for land data.
Accordingly, it is an object of the present invention to provide a method and system for modeling the energy of multiple reflection signals in land data, which can be used to effectively model the primary reflection signals.
It is another object of the present invention to provide a method and system for modeling the energy of multiple reflection signals of land data without the detrimental effects associated with conventional approaches, such as data smearing.